The current trend in microelectronic device fabrication is to manufacture smaller and faster microelectronic devices for computers, cell phones, pagers, personal digital assistants, and many other products. All microelectronic devices generate heat, and rejection of this heat is necessary for optimum and reliable operation. As the speed and capacity of microelectronic devices has increased, the integrated circuitry of the devices has become smaller and more closely spaced, thereby generating more heat. Moreover, the cooling space within the microelectronic devices has become smaller. Accordingly, heat dissipation has become a critical design factor.
FIGS. 1A-1C schematically illustrate an existing method for dissipating heat from devices formed on a wafer 110. FIG. 1A is a schematic side view of the wafer 110, and FIG. 1B is a schematic side view of the wafer 110 thinned, for example, in accordance with the procedures disclosed in U.S. Pat. No. 6,180,527, assigned to the assignee of the present invention and incorporated herein by reference. Thinning the wafer 110 increases the surface area per unit volume of the wafer 110, and therefore the ability of the wafer 110 to reject heat. FIG. 1C illustrates a microelectronic device 100 including a portion of the diced wafer 110, such as a microelectronic die 140, and a heat sink 130 attached to the die 140. In operation, the heat sink 130 absorbs heat from the die 140 and dissipates the heat into the ambient air. In one embodiment, a cooling fan can be added to force air past the heat sink 130.
Another method for dissipating heat from the die 140 includes attaching a heat pipe (not shown) to the surface of the die 140. A heat pipe typically includes a closed, evacuated vessel with a working fluid inside. One end of the heat pipe is positioned to absorb heat from the die 140. The heat causes the fluid in the heat pipe to vaporize and create a pressure gradient in the pipe. This pressure gradient forces the vapor to flow along the heat pipe to a cooler section where it condenses, giving up its latent heat of vaporization. The cooler section of the heat pipe then dissipates the heat into the ambient air. The working fluid then returns to the end of the heat pipe proximate to the die 140.
The foregoing heat dissipation methods have several drawbacks. For example, attaching a heat sink, heat pipe, and/or cooling fan to the microelectronic device may substantially increase the weight and/or size of the device. Furthermore, the limited contact area between the die and the heat sink or heat pipe may limit the heat transfer between the devices.